As well known in the art, acrylic acid is produced from propylene by gas-phase oxidation method. This method for producing acrylic acid by oxidizing propylene, includes a first stage reaction for oxidizing propylene into acrolein and a second stage reaction for oxidizing the acrolein into acrylic acid which are conducted under different oxidation conditions from each other. Therefore, as the above method, there are known a two-stage oxidation process in which the respective reactions are performed using different kinds of catalysts or separate reactors, and a one-stage oxidation process using a single reactor filled with plural kinds of catalysts.
FIG. 1 shows an example of a flow chart for producing acrylic acid by the two-stage oxidation process. In the process shown in FIG. 1, propylene, steam and air are fed through the first reactor and the second reactor filled with molybdenum-based catalysts, etc., and subjected to two stage oxidation reactions therein to obtain an acrylic acid-containing gas. The thus obtained acrylic acid-containing gas is fed to a condensation column (quench column) where the gas is contacted with water to prepare an aqueous acrylic acid solution. The aqueous acrylic acid solution is fed to an extraction column where acrylic acid is extracted with an appropriate extractant added thereto, and the obtained extract solution is then fed to a solvent separation column to separate the extractant therefrom. Next, the obtained solution is fed to an acetic acid separation column where acetic acid is removed therefrom to prepare crude acrylic acid. The thus prepared crude acrylic acid is then fed to a rectifying column to separate by-products from the crude acrylic acid, thereby obtaining purified acrylic acid.
Meanwhile, in recent years, in order to recover acrylic acid from the aqueous acrylic acid solution, instead of the above solvent extraction method using the extractant, there has been used an azeotropic separation method in which the aqueous acrylic acid solution is distilled using water and an azeotropic solvent to distil off an azeotropic mixture of water and the azeotropic solvent from a top of an azeotropic separation column and recover acrylic acid from a bottom thereof.
Methacrylic acid is produced by gas-phase oxidation reaction of isobutylene. In the case where methacrylic acid is produced by two-stage oxidation method, isobutylene is oxidized into methacrylic acid through methacrolein.
The gas-phase oxidation reaction of propylene or isobutylene is conducted within an oxidation reactor filled with an oxidation catalyst. The oxidation reactor includes a container-like reactor body, and a manhole nozzle projecting from the reactor body. The manhole nozzle is closed by a lid fitted to a tip end thereof upon normal operation of the reactor. The lid is opened upon inspection of an inside of the reactor, replacement of the catalyst or the like.
Also, although not shown in the figure, in order to fit various sensors or gauges, etc., to the reactor, the reactor body may be frequently provided with measuring device-mounting nozzles to which the sensors or gauges may be fitted. Further, the reactor may also be frequently provided with a small diameter nozzle serving as an opening for inspection of an inside of the reactor.
When viewed from the inside of the reactor body, portions where the above respective nozzles projecting from the reactor body are provided, form recesses partially depressed from an inner surface of the reactor. These recesses tend to cause undesirable retention of a reaction gas. For this reason, in the above conventional reactors, an easily-oxidizable substance gas such as (meth)acrolein tends to be retained inside of the respective nozzles and automatically oxidized therein, thereby causing unstable oxidation reaction of the gas.
Since these nozzles projecting from the reactor body tend to be cooled by outside air, etc., the easily-polymerizable substances such as (meth) acrolein and (meth)acrylic acid may be liquefied and retained therein to produce polymers thereof. As a result, the nozzles tend to be clogged, so that it may become extremely difficult to open the nozzles and manholes upon terminating operation of the reactor, etc.
On the other hand, as a method of sampling and analyzing a gas containing the above easily-polymerizable compound, there are known a method using a reaction product gas sampling tube, a gas-liquid separation method, a sampling container method, a sensor method or the like.
Among these methods, the method using a reaction product gas sampling tube is industrially advantageous from viewpoints of simple facilities and low costs. As the reaction product gas sampling tube, there may be mainly used a stainless steel sampling tube around which an electric heater or a steam trace is fitted.
However, the stainless steel sampling tube around which the electric heater is fitted, has the following problems. That is, although portions at which thermocouples are provided for temperature control are maintained at a set temperature, other portions of the sampling tube tend to undergo undesirable temperature distribution depending upon winding conditions of the electric heater or heat-retention conditions. As a result, easily-condensable substances tend to be condensed at the low-temperature portions, so that an analysis accuracy thereof tends to be deteriorated. Further, acrylic acid whose polymerization is inhibited in a gas state but is promoted in a liquid state, tends to condensed and polymerized at the low-temperature portions formed due to the temperature distribution. If the operation of the sampling tube is continued, the sampling tube tends to suffer from clogging, resulting in failed analysis. In particular, since a longer sampling tube undergoes a larger temperature distribution, the use of such a longer sampling tube tends to further deteriorate the analysis accuracy, and promote clogging thereof. In addition, at the time other than the analysis, the reaction production gas is retained within the sampling tube, so that the easily-condensable and easily-polymerizable substances tend to be condensed, polymerized and adhered to an inside of the sampling tube, also resulting in problems such as clogging thereof.
In the case where the steam trace is wound around the stainless steel sampling pipe, it may be difficult to uniformly wind the steam trace around the thin sampling tube, resulting in occurrence of temperature distribution in the sampling tube, especially formation of the low-temperature portions therein. Therefore, the latter method using the steam trace has similar problems to those of the former method using the electric heater, such as deteriorated analysis accuracy, tendency of causing clogging in the sampling tube and difficulty in long-term continuous operation thereof. Thus, both of the above methods are unsatisfactory from industrial viewpoints.
In Japanese Patent Application Laid-open (KOKAI) No. 8-259488, to solve the above problems, there has been proposed the sampling tube whose inner wall is made of a fluorine-based resin. The sampling tube whose inner wall is made of a fluorine-based resin is more effective to prevent a condensate of the easily-condensable substances from adhering to an inside of the sampling tube as compared to the stainless steel sampling tube. However, the sampling tube whose inner wall is made of a fluorine-based resin is still unsolved as to the problem concerning temperature distribution, resulting in low analysis accuracy. In addition, the sampling tube whose inner wall is made of a fluorine-based resin is also still unsolved as to such a problem that at the time other than the analysis, the reaction production gas is retained within the sampling tube, so that the easily-condensable and easily-polymerizable substances tend to be condensed, polymerized and adhered to an inside of the sampling tube, thereby causing the clogging of the sampling tube.
An object of the present invention is to provide an oxidation reactor that is free from recesses formed on an inner surface of a reactor body thereof so as to conduct a stable oxidation reaction therein, as well as a process for producing (meth)acrylic acids using the oxidation reactor.
Another object of the present invention is to provide an oxidation reactor that is free from undesirable retention of gas at recesses formed on an inner surface of a reactor body thereof so as to conduct a stable oxidation reaction therein, as well as a process for producing (meth)acrylic acids using such an oxidation reactor.
A further object of the present invention is to provide an on-line analysis process for analyzing an easily-polymerizable compound by introducing a gas containing the easily-polymerizable compound into an analyzing apparatus through a reaction production gas sampling tube in which the reaction production gas is prevented from being condensed and polymerized in the sampling tube, thereby ensuring a long-term stable analysis with a high efficiency.